Money is often based on metals but need not be. It could be pieces of polished shell, beads of volcanic glass, small plates of jade or other semi-precious stone, thick dried leaves of a certain plant, discs of bone or horn. Your medium of exchange just needs to be fairly rare, possibly pretty, and considered valuable. And, of course, portable. In ancient Sumeria, they issued letters of credit in the form of clay tablets. So even mud can be money, depending on the culture.